What might cause a sudden increase in idle speed?

[Bratina]

For various reasons I haven't run my engine since May 14th. At that time it ran ideally, with no issues and idle speeds around 700.

Yesterday I started the engine and noted that it idled much higher - around 1100 rpm (in or out of gear), and would start to lug if I tried to adjust down the idle speed by moving the throttle stop.

I have changed nothing: no modifications to the boat whatsoever since I last ran the engine.

It ran well otherwise - moving in and out of gear without issue, and ran for around 40 minutes at approximately 2000 rpm (WOT was around 2500) with good temperatures and oil pressure, no blow-by etc.

I'm puzzled: any recommended diagnostics?

[Dave Neptune]

Bratina, if nothing was changed about all that would cause an increase in idle speed is an air leak. Check the base gasket of the carb and the scavange tube first. It does not take much of a leak.

If the engine was running a bit rougher and faster it may be time to pressure check the manifold.

Could also be the centrifugal advance is stuck advanced.

Dave Neptune

[Bratina]

Bratina, if nothing was changed about all that would cause an increase in idle speed is an air leak. Check the base gasket of the carb and the scavange tube first. It does not take much of a leak.

If the engine was running a bit rougher and faster it may be time to pressure check the manifold.

Could also be the centrifugal advance is stuck advanced.

Dave Neptune

Thanks Dave.

I'll check the carb and scavenge tube first for air leaks. The fuel filter is new, but added before my previous outing, however I'll confirm that it is seated well and not leaking air.

The engine didn't run rough, but I'll add a pressure test to the list.

If the c-adv was stuck open, would the engine continue to rev up/down freely (albeit at a higher idle)? It's high time I ripped open the distributor and took a good look anyway. It's been a case of 'its not broken, don't fix it', and is untouched in a couple of years.

If I take off the cap, rotor etc to check and lube the c-adv (but without removing the distributor from the engine), will the engine's timing be preserved when I reassemble everything?

[Dave Neptune]

If the adv is stuck the engine will still rev up and down just not as smooth or powerful.

Just pop the cap off and give the rotor a twist it should move about 17 degrees and return back when released. Don't worry about the degrees just is it somewhat smooth in both directions.

If you don't mess with the points the timing should stay put. If you R&R the "plate" to get to the C-advance the timing will change only slightly due to the points adjustment.

Dave Neptune

[Bratina]

If the adv is stuck the engine will still rev up and down just not as smooth or powerful.

Just pop the cap off and give the rotor a twist it should move about 17 degrees and return back when released. Don't worry about the degrees just is it somewhat smooth in both directions.

If you don't mess with the points the timing should stay put. If you R&R the "plate" to get to the C-advance the timing will change only slightly due to the points adjustment.

Dave Neptune

I ran the engine again today, with the same approximate idle in/out of gear. It continues to run extremely well, apart from the high idle speed. WOT in gear was around 2,500rpm.

1. I checked the rotor, and it turns smoothly approximately 1/2" inch (so about the right angle). No resistance / grinding. I think the adv is good

2. I removed/replaced the carb, and can't see a leak between the halves / at the manifold. I used new gaskets.

I reinstalled everything and it fired up well, and continued to run high idle but smoothly. Frustrating.

There was a little pitting in the cap. I'd like to replace it before a trip this weekend. I always support our kind host, but in this case if any Canadians know of a brick and mortar store (and part #) where I can pick up a cap (and probably worth doing the points and rotor at the same time?), I'd appreciate it. I'm near Toronto.

I tried to pull off the rotor since I was there, and it wouldn't budge. I didn't use a whole lot of force: should it be difficult to remove? There are the remains of what looks like painters tape (!), which might be jamming the rotor.

Finally, the only difference in my boat now vs. a month ago is that power is off to my dock due to flooding. My batteries are always plugged in and charged up, apart from now. Is there a chance that the high idle speed is related to alternator activity?

[JOHN COOKSON]

Have you measured the dwell? Dwell is the sort or "summary measurement" to see if the points are set correctly or if there is abnormal wear with the points or distributor.
Once you are reasonably certain the ignition and fuel side of the engine are correct you will be able to get the idle correct by adjusting both the idle speed and idle mixture. You will have to sort of "play" one against the other.
Have you tried turning the idle mixture control needle lightly to the stop then backing it out 1 1/2 turns. This is a stretch but maybe there was a bit of crud in the carburetor idle circuit that became dislodged and now the idle has changed.

TRUE GRIT

Edit: The rotor should slide on and off easily. The tape makes me think someone used an incorrect rotor and needed the tape to hold it in place.

[thatch]

Since I haven't seen it mentioned yet, I'll ask the question, is there a PCV valve involved? If there is, this would change the whole line of questioning.
Tom

[Peter]

There was a little pitting in the cap. I'd like to replace it before a trip this weekend. I always support our kind host, but in this case if any Canadians know of a brick and mortar store (and part #) where I can pick up a cap (and probably worth doing the points and rotor at the same time?), I'd appreciate it. I'm near Toronto.

You may find the following link useful for part numbers - http://www.alberg30.org/maintenance/...4/A4parts.html

On another note, Marvel Mystery Oil can be had in Canada from Costco online - https://www.costco.ca/Marvel-Mystery...100244472.html

You have to order it but they deliver it at no charge. I ordered some on Tuesday and it is scheduled for delivery today.

Peter

PS - apologies to our hosts but with the exchange rate, shipping and import fees...

[Bratina]

Have you measured the dwell? Dwell is the sort or "summary measurement" to see if the points are set correctly or if there is abnormal wear with the points or distributor.
Once you are reasonably certain the ignition and fuel side of the engine are correct you will be able to get the idle correct by adjusting both the idle speed and idle mixture. You will have to sort of "play" one against the other.
Have you tried turning the idle mixture control needle lightly to the stop then backing it out 1 1/2 turns. This is a stretch but maybe there was a bit of crud in the carburetor idle circuit that became dislodged and now the idle has changed.

TRUE GRIT

Edit: The rotor should slide on and off easily. The tape makes me think someone used an incorrect rotor and needed the tape to hold it in place.

Thanks for the reply. I haven't measured dwell, but I'm not too sure how it would have changed substantially during a month layup? I don't have a meter, but I can look to borrow one if needed.

I've set and reset the idle needle a couple of times, but didn't cause any change when back out 1.5 turns. Would a blocked idle passage result in high idle speeds? I'll take apart and clean the carb to be sure.

I think you're right about the rotor - I'll try a little extra force.

[Bratina]

Since I haven't seen it mentioned yet, I'll ask the question, is there a PCV valve involved? If there is, this would change the whole line of questioning.
Tom

Thanks Tom - no PCV valve, just a standard stock slash tube.

On that note I took the flame arrestor off to ensure the choke was opened fully when running. It is, but 1: the screws aren't captive (DAMHIK) and 2. there is a remarkable and startling amount of suction if you place your palm over the intake..

[Bratina]

You may find the following link useful for part numbers - http://www.alberg30.org/maintenance/...4/A4parts.html

On another note, Marvel Mystery Oil can be had in Canada from Costco online - https://www.costco.ca/Marvel-Mystery...100244472.html

You have to order it but they deliver it at no charge. I ordered some on Tuesday and it is scheduled for delivery today.

Peter

PS - apologies to our hosts but with the exchange rate, shipping and import fees...

Thank you Peter, that's very helpful. I promise to continue to order 'big' and non-urgent items from our wonderful hosts.

I also didn't know about MMO from Costco! I've seen it infrequently at Wal-Mart, but I can stock up now. Thanks.

[JOHN COOKSON]

I've set and reset the idle needle a couple of times, but didn't cause any change when back out 1.5 turns. Would a blocked idle passage result in high idle speeds? I'll take apart and clean the carb to be sure.
.

If you can't control the idle with the idle mixture adjustment screw then something else is controlling the idle. When you figure out what that something else is you will have the answer to your problem. Taking the carb apart and cleaning up the inwards and reaming out the passages sounds like the thing to do.
A crumpled rag under the work area will catch the parts you are going to drop.

TRUE GRIT

[Dave Neptune]

Bratina, are you sure the "throttle arm" is against the "idle speed stop screw"? Could be in the cable.

If the "idle mixture screw" makes no difference you are on the transfer to "hi speed metering" and not on the idle circuit. Something is amiss.

Just maybe things weren't as "right" as you thought with the carb. How is the power compared to your last few outings? And how long since any type of carb work or adjustment?

Is it easy to pull one of the plugs and give a color reading or best a picture of one or more. 1 & 4 work best for comparison.

Dave Neptune

[Bratina]

Bratina, are you sure the "throttle arm" is against the "idle speed stop screw"? Could be in the cable.

If the "idle mixture screw" makes no difference you are on the transfer to "hi speed metering" and not on the idle circuit. Something is amiss.

Just maybe things weren't as "right" as you thought with the carb. How is the power compared to your last few outings? And how long since any type of carb work or adjustment?

Is it easy to pull one of the plugs and give a color reading or best a picture of one or more. 1 & 4 work best for comparison.

Sorry, I was very unclear in my post: the idle mixture screw has an effect, it’s just that adjusting it in and back out didn’t resolve any problems. The engine bogs down and stalls if I lean it out too far. I'm able to lower idle speed to around 1,050rpm by balancing the idle and set screw, but no lower, and I'm at ~2 full turns out.

The engine generates a little smoke at 2,500rpm.

I pulled plugs #1 and #4. I can't upload a picture from here, but #1 had slight fouling, but an overall tan colour. #4 was sooty and had moderate fouling.

I took apart the carb and gave it a thorough cleaning (including reaming out the idle ports and using compressed air to blow out passages). There was some dirt in the carb, including a gum on the main jet. The clean carb had no effect on idle speed when I reinstalled it last night.

The two halves don't meet at the front and, not wanting to sand further, I've always used a thin bead of permatex red on the outside of the carb. Normally I'd say that was a potential concern, but the engine ran great all last year, and great this year until recently.

I have an inline polishing filter (from Moyer) which is a couple of years old. I'm starting to think fuel starvation, so this morning I removed it and ran the hose right from the mechanical pump to the carb. A little gas escaped and so I didn't test the engine again yet.

Power is good, it doesn't stall going in/out of gear and, apart from moving the boat a little faster at 'idle' the issue isn't causing me problems in use. I do worry that I don't understand the root cause, and wonder if I'm damaging the gearbox when I shift into forward at ~1,100rpm?

Any suggestions for further diagnostics I can try if removing the filter effects no change to the idle speed?

[Dave Neptune]

Carb internnal air leak.

Bratina, the "idle mixture screw" is an AIR adjustment so screwed out is Lean and screwing in is RICH.

RE the halves of the carb. There is a piece in the middle of the bowl called the emulsion well where most of the mixing of air into the fuel is done for the hi speed AND it also feeds fuel to the idle circuit where the "idle mixture screw" adds the air to the idle circuit which is adjustable. Now where this well seals against the top half is CRITICAL that there be a good seal. If not sealed it will leak air and the carb will not work properly at all!!!!!

I think this just ma be your issue here.

Dave Neptune